Nip-fed sheet media transport systems using paired rollers are widely used in various printing applications. In a nip-fed system, a drive roller is pressed against a backing roller to form a nip and provides drive motion at the nip. A nip-fed transport can be engineered to perform with a suitable degree of accuracy in devices such as printers and office copiers. However, conventional nip-fed media transport mechanisms do not provide sufficient precision for imaging applications that require high resolution. For example, many types of medical imaging apparatus print onto a sheet of recording medium at resolutions well exceeding 600 dots per inch. For such devices, a sheet media transport must provide extremely accurate motion when moving the sheet through the image recording mechanism. This problem becomes even more pronounced with full-sheet imaging, in which little or no margin is to be provided at the leading or trailing edges of a sheet. As is well appreciated by those skilled in media transport arts, the dynamics of handling and urging a sheet of recording medium through a printing mechanism can be much more complex at the leading and trailing edges that along more central portions of the sheet.
Dual nip apparatus provide advantages where it is necessary to provide more precise motion control for sheet media. By using two pairs of rollers in series along the transport path, a more stable sheet media transport is provided, since the motion of the medium is controlled through at least one nip at any point during the image recording process. FIG. 1 shows, in schematic form, a conventional dual nip transport apparatus 10 as used for a sheet of recording medium 12. In the travel path, recording medium 12 is fed through an entrance nip 14 formed between an entrance drive roller 16 and a pressure roller 18, then through an exit nip 24 formed between an exit drive roller 26 and a pressure roller 28. Image data is recorded by a printhead 56, or other type of write head or recording head, that records the image onto recording medium 12 as it is progressively scanned through an imaging area 20 between entrance nip 14 and exit nip 24. Typically printhead 56 is a scanned point source, such as a laser or other source of electromagnetic radiation, that is scanned across recording medium 12 in a direction orthogonal to the direction of motion Q. In order to provide uniform speed with dual nip media transport apparatus 10, it is necessary to couple the speed of entrance drive roller 16 at entrance nip 14 with the speed of exit drive roller 26 at exit nip 24. The conventional method for coupling rotation of entrance and exit drive rollers 16 and 26 is using a belt 22, as shown in FIG. 1.
Dual nip transport devices such as dual nip transport apparatus 10 in FIG. 1 perform sufficiently well for many types of imaging applications. However, dual nip transport arrangements using conventional design approaches fall somewhat short of performance levels needed for high-resolution medical imaging applications. One notable type of problem relates to transitions as the sheet medium moves through the entrance and exit nips, particularly at leading and trailing edges of a sheet. At certain points in the transport cycle, the sheet medium goes through a transition between being held in one nip and being held in both nips. As the sheet edge enters or exits a nip, any abruptness in handling can cause a corresponding effect on the image recording operation.
Thus, it can be seen that there is a need for a transport mechanism that provides precision handling of single sheet media at a constant transport speed, allowing full sheet scanning and imaging from leading to trailing edge and minimizing the effects of transitions at leading and trailing edges.